1. Field of the Invention
This invention relates to methods for hydrolyzing oat husks to provide D-(+)-xylose.
2. Description of the Prior Art
D-(+)-xylose and its hydrogenation product, xylitol, are of considerable industrial significance. For example, xylose can be employed for various purposes in the foodstuffs industry, while xylitol has proved to be a very good sweetener for diabetics. Varieties of deciduous timber, such as beech and chestnut, are used almost exclusively as the starting material for the industrial manufacture of xylose. The yields are about 10-12% (compare, for example, German Pat. No. 912,440). It is a significant disadvantage of these processes that the wood material which remains, so-called cellolignin, has hitherto been incapable of practicable industrial utilization and that the process mentioned only gives moderate yields of xylose.
German Pat. No. 834,079 has disclosed the production of xylose from oat husks. Oat husks contain about 38% of pentosan while, for example, beechwood and birchwood only contain 24-25 % and maize cobs contain about 28% of pentosans. In this process the oat husks are heated to boiling with 0.08% strength ammonia or are extracted with benzene/alcohol. Thereafter the usual hydrolysis under pressure is carried out with 0.2 to 0.5% strength H.sub.2 SO.sub.4 at 125.degree. C. Further working up is not carried out. In the pretreatment with NH.sub.3, 4 kg of NH.sub.3, as an 0.08% strength solution, are used per 1,000 kg of oat husks. However, 17 kg of NH.sub.3 would be necessary to split off the acetic acid. Furthermore, under the conditions mentioned in German Pat. No. 834,079, it is likely that hardly any splitting off, and hence removal, of the acetic acid, which accounts for approximately 6% of the weight of the oat husks, takes place.
The pending U.S. Patent applicaton Ser. No. 523,456 of Hermann Friese, filed Nov. 13, 1974, discloses a method for the preparation of xylose solutions from oat husks which is characterized in that the hydrolysis is carried out upon the oat husks with alkali metal hydroxide or alkali metal chlorite in a first stage to remove substantially all of the chemically bound acetic acid originally present therein and hydrolyzing the acetic acid-free oat husks with a mineral acid in a second stage to provide a residue containing lignin and an acidic solution of D-(+)-xylose. Using the method, the xylose can be recovered as such or converted in situ to xylitol. The solid residue by-product of the aforesaid process for the hydrolysis of oat husks, following removal of the lignin content thereof, is also readily convertible to cellulose.